Field of the Invention
The invention concerns a method to acquire a magnetic resonance data set of an acquisition area in a subject by radial scanning (data entry) along a fixed number of spokes in k-space, the spokes respectively being described by an angle, wherein the acquisition area in the spatial domain deviates from a circle in a shape that can be described by the set of angles.
In addition, the invention concerns a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Image acquisition methods in which a radial scanning of k-space (entry of acquired raw data at respective points in k-space) takes place are known. The scanning in the image plane takes place along spokes of a defined angle proceeding through the k-space center. If equally distributed spokes are used over the entire angle interval of 360°, a circular acquisition area results in the spatial domain in the image plane. In an article by K. Scheffler et al., “Reduced Circular Field-of-View Imaging”, MRM 40:474-480 (1998) it was shown that non-constant spoke density functions allow the definitions of non-circular acquisition areas over the angles. Further aspects are discussed in this context to an article by P. Larson et al., “Isotropic Field-of-Views in Radial Imaging”, IEEE TMI 27(1), Page 47-57, 2008.
If radial scanning is used for subjects with non-circular dimensions, given undersampling markedly more severe undersampling occurs in the direction of the larger dimension of the subject. With knowledge of the cited articles, this can be compensated by adaptation of the spokes to the subject dimension. As mentioned, this shape of the acquisition area in the spatial domain that deviates from a circle can be described just as well by a non-constant, angle-dependent spoke density in k-space as by the angles of spokes given a fixed number of spokes.
If dynamic processes in the acquisition area are to be detected, it has been proposed, for example, in an article by Stefanie Winkelmann et al., “An Optimal Radial Profile Order Based on the Golden Ratio for Time-Resolved MRI”, IEEE TMI 26 (1), Page 68-76, 2007, to use a trajectory based on the golden angle for the radial scanning. The angles thus are not measured in their spatial order; rather, an acquisition position is respectively associated with the individual spokes in an acquisition order, wherein the angles of successive spokes to be acquired are differentiated by a golden angle, wherein the golden ratio of 180° is preferably used which lies at 111.25°, for example. In this regard, in the cited article it was established that such a trajectory based on the golden angle is provided for every arbitrary subset of successive spokes, results in nearly an equal distribution of the spokes. An exactly equal distribution results if the fixed number of spokes is a Fibonacci number. For example, after the measurement this enables time intervals to be defined so that a tradeoff can be freely chosen between temporal and spatial resolution and subsampling artifacts. Such a trajectory based on the golden angle also allows subsets of the acquired spokes to be combined arbitrarily. However, such a trajectory based on the golden angle is only reasonably conceivable for circular acquisition areas (fields of view); for non-circular acquisition areas, a constant angle increment no longer delivers these desired properties.